The present invention relates to inspection apparatus for optically inspecting the surface of a workpiece, and also to a surface illumination system for such inspection apparatus. The invention is particularly useful in a system for the high speed, automated, optic inspection of printed circuit boards, wafers and the like, for detecting faults which require reworking, and therefore the invention is described below with respect to this application, but it will be appreciated that the invention, or features thereof, could advantageously be used in other applications as well.
The conventional inspection apparatus for optically inspecting the surface of a workpiece, such as a printed circuit board or wafer, in order to detect faults typically includes a memory for storing reference data relating to the desired features of the workpiece surface, an illumination system for illuminating the workpiece surface, an optic sensor for sensing the light reflected from the illuminated workpiece surface and for outputting electric signals corresponding thereto, and a processor including logic circuitry for analyzing the electric signals outputted by the optic sensor for comparing them with the data stored in the memory, and for providing an indication of any discrepancies with respect thereto indicating a defect in the inspected workpiece surface. The reference data relating to the desired features of the workpiece surface to be compared with the sensed workpiece surface, may be in the form of stored images of the desired workpiece surface, or of stored design rules for the design of such workpiece surface.
Since the workpiece surfaces are not perfectly flat, but rather exhibit some degree of surface relief such as grooves, scratches, or angled surfaces, it would be desirable that the illumination system include a "Lambertian" diffuser, namely a perfect diffuser effective to cause the intensity of reflected radiation to be indepedent of direction. Such a diffuser would produce spatial uniformity of the light (i.e., a uniform "sky of illumination") above the workpiece and thereby eliminate shadows caused by the relief in the workpiece surface. However, Lambertian diffusers are extremely wasteful of light, and therefore such a diffusing surface would require an extremely intense light source and extremely high power, or would substantially slow-down the operation of the inspection apparatus in order to obtain workable signals having the required signal-to-noise ratio.
Our prior U.S. patent application Ser. No. 07/370,064 filed June 21, 1989 (which is a continuation of application Ser. No. 07/150,332 filed Jan. 28, 1988, and is based on Israel Patent Application 81459 filed Feb. 2, 1987) discloses an illumination system particularly useful for an optic inspection apparatus, which system provides what may be termed "Quasi-Lambertian" illumination. More particularly, that patent application describes an illumination system for optic scanners comprising darkfield illumination means, and combined brightfield-darkfield illumination means for illuminating a line to be scanned on the surface of the workpiece to be inspected. The darkfield illumination means comprises first and second light guides producing, at their output ends, elongate, narrow light sources extending parallel to and on opposite sides of the line to be scanned; and a beam concentrator for each of the light guides located to produce an at least approximate image of its respective light source on the line to be scanned on the surface of the workpiece. Each of the beam concentrators further includes a reflector focussing member and an aberration plate for slightly defocussing or diffusing the image of its respective light source on the line to be scanned. In addition, the brightfield illumination means also include a light guide producing, at its output end, an elongate, narrow light source, and a reflector focussing member and aberration plate for producing an approximate image of its respective light source on the line to be scanned.
Illumination systems constructed in accordance with the system illustrated in our prior application Ser. No. 07/150,332 have been found to be highly effective in eliminating shadows caused by the relief in the workpiece surface, while at the same time to be very economical in the power required and in the light produced, such that the inspection apparatus can be operated at a substantially high speed while still obtaining workable signals having the required signal-to-noise ratio.
An object of the present invention is to provide an improved illumination system of the foregoing type which enables the system to be considerably miniaturized so as to take up a small fraction of the space of the system described as a preferred embodiment in our patent application Ser. No. 07/370,064. Another object of the present invention is to provide an improved illumination system of the foregoing type in which the solid angle of illumination is considerably increased as compared to the system described as a preferred embodiment in the above-identified patent application.
According to the present invention, there is provided an illumination system for optic scanners as described in our patent application Ser. No. 07/370,064, wherein the darkfield illumination means comprises an elongate lenticular lens sheet (instead of an aberration plate) between each of the first and second light sources and their respective reflector focussing members effective to spread the light over their respective reflector focussing members.
Each lenticular lens sheet is one having an array of cylinder lenses aligned perpendicularly to its respective narrow light source. The use of such an array of cylinder lenses, rather than a conventional aberration plate such as a ground glass diffuser, enhances the light efficiency of the system. By aligning the cylindrical lenticles perpendicularly to the light source, the light spreading is effected only along the axis of the cylinder focussing elements, and thereby better ensures adequate angular coverage around an axis perpendicular to the scanned line.
According to further features in the preferred embodiment of the invention described below, the brightfield illumination means comprises a third light source mounted laterally of the optic axis; a refractive (instead of reflective) focussing member for producing an approximate image of the third light source on the line to be scanned on the surface of the workpiece; another lenticular lens sheet for spreading the light from the third light source over the refractive focussing member; and a beamsplitter aligned with the optic axis for reflecting the light from the focussing member towards the workpiece, and for conducting therethrough both the darkfield and brightfield illumination reflected from the workpiece to the optic scanner.
According to still further features in the described preferred embodiment, each of the reflector focussing members is a concave reflector which subtends an arc of 50.degree.-60.degree. (preferably about 50.degree.) on each side of the optic axis, and the two reflector focussing members are spaced from each other 2.degree.-10.degree. (preferably about 5.degree.) on each side of the optic axis.
An illumination system constructed in accordance with the foregoing features has enabled the system to be miniaturized such as to occupy a space of only about one-fifth that of the commercial form of the system described as a preferred embodiment in our prior patent application Ser. No. 07/370,064. Such system also provides a considerably larger solid angle of illumination, i.e., about 110.degree., as compared to about 60.degree. in the commercial form of that system.
The present invention also provides inspection apparatus for optically inspecting the surface of a workpiece, comprising an illumination system as described above for illuminating the surface of the workpiece. Such inspection apparatus also comprises a memory for storing data relating to the desired features on the workpiece surface; an optic sensor for sensing the light reflected from the illuminated workpiece surface and for outputting electrical signals corresponding thereto; and a processor including logic circuitry for analyzing the electric signals outputted by the optic sesnor, for comparing them with the data stored in the memory, and for providing an indication of the discrepancies with respect thereto indicating a defect in the inspected workpiece surface.
When an illumination system as described above is used in such inspection apparatus, the objective lens of the optic inspecting system may be much more closely located with respect to the workpiece. This greatly simplifies the lens design, and also effects substantial savings in the light and power requirements in order to obtain the necessary signal-to-noise ratio for any scanning speed.
Further features and advantages of the invention will be apparent from the description below.